It has heretofore been proposed to employ so-called time domain reflectometry techniques to measure or detect the level of material in a storage vessel. In general, this technique involves placement of a conductive transmission line probe in the vessel at an orientation to be contacted by material in the vessel. Microwave pulses of short duration are periodically launched onto the transmission line probe, typically in a downward orientation through air toward the material surface. When the pulses encounter an electrical discontinuity, such as the change in dielectric constant at the interface between the air and the material, a portion of each energy pulse is reflected back along the transmission line probe to detection electronics. Time-delayed gating techniques are employed in a process referred to as equivalent time sampling to locate the position of the reflecting discontinuity along the transmission line probe, and thereby to determine the level of the material surface with respect to the probe.
Although the material level sensing technique so described has overcome problems and difficulties theretofore extant in the art, further improvements remain desirable. For example, systems employing this technology have been implemented for continuous level measurement--i.e., measurement of material level as a continuous function throughout a desired range. However, there remains a need for a point-level measurement system employing this technology that is less expensive to manufacture and easier to implement than use of continuous-level technology coupled with a point-level threshold detection. It is one object of the present invention to provide a material point-level detection system and method that address this need in the art.
Another deficiency in the art surrounds the mounting of the transmission line probe to the vessel. In typical situations, the electronics are mounted within a housing, and the housing is mounted to the wall of the vessel such that the transmission line probe extends into the vessel for contact with the material. The transmission line probe typically takes the form of a coaxial transmission line, in which the probe rod forms the center conductor and the vessel wall forms the outer conductor. It is important that the characteristic impedance of the transmission line probe match that of the interconnection to the electronics until the probe rod is within the vessel, so that the vessel wall can form a launch plate for the probe rod without undesirably bleeding energy from the probe rod or causing spurious reflections due to impedance mismatch. It is therefore another object of the present invention to provide an electronics/probe interconnection arrangement that efficiently transfers pulsed microwave energy from the launching electronics to the probe without excessive energy loss or spurious reflections. A further object of the present invention is to provide a system for continuous measurement of material level in a vessel in which the upper and lower level limits for the continuous measurement may be readily reprogrammed by an operator. Yet another object of the present invention is to provide an inexpensive and reliable method of making coaxial connection to a transmission line probe in a point or continuous level measurement system of the described character.